1. Field of the Invention
This invention is in the field of hot spraying metal alloys, particularly in the form of wires to produce a coating which is generally non-porous and contains oxide in the form of particles and films with the thickness not exceeding approximately 2 microns distributed in the alloy layer so that grain growth in the alloy layer is prevented while at the same time the desired diffusion of aluminum atoms from the interior of the layer to the surface is permitted.
2. Description of the Prior Art
Methods of applying metallic layers to substrates by use of flame spraying and the like are already known in the art. Generally, such layers are porous and have large oxide contents which may prevent them from welding together to form non-porous alloy layers.
Alloys of the iron-chromium-aluminum type containing 1 to 12% aluminum and 10 to 30% chromium have long been used as electric resistance elements since they resist oxidation at very high temperatures on the order of 1300.degree. C. and above. Such alloys are manufactured by melt-metallurgical processes in the forms of wires or strips which can be formed into resistor elements.
It has long been desired to be able to produce dense extended layers of such iron-chromium-aluminum alloys, but it has not been proven practical to produce such layers either by melt-metallurgical processes or by powder metallurgical processes. There is a mention in the technical literature (Korrosion 18, Verlag Chemie GmbH, Weinheim/Bergstr., Germany 1966) as to the desirability of protecting steel with a layer of "Kanthal" alloy which is the trademark for an iron-chromium-aluminum cobalt alloy. The layer in the reference has a thickness of about 300 microns.
French Pat. No. 1,172,867 mentions the use of a "Kanthal" alloy among 12 alloys as thin binding layers for oxide coated bodies. However, no further information is given with regard to the composition, structure or the properties of the layers. The composition of the wire to be sprayed according to the French patent is 2% cobalt, 6% aluminum, 23% chromium, and 69% iron, but the composition of the final layer is not defined. The composition of the layer is not predictable because the structure and the analysis of the alloy is subject to considerable changes under the influence of high temperature and other factors which prevail during spraying.